1. Nottingham Transportation Engineering Centre [NTEC], Dept. of Civil Engineering, Univ. of Nottingham, Nottingham NG7 2RD, GB
2. GITECO Research Group, Universidad de Cantabria. E.T.S. Ingenieros de Caminos, Canales y Puertos de Santander, Av. de los Castros s/n, 39005 Santander, ES
3. Nottingham Transportation Engineering Centre [NTEC], Dept. of Civil Engineering, Univ. of Nottingham, Nottingham NG7 2RD, GB (Corresponding author), e-mail: gordon.airey@nottingham.ac.uk